1. Field of Invention
The invention relates to a film blowing head for the production of film tubing comprising a single or multi-layer film. Such film blowing heads are known.
2. Description of the Prior Art
Generally they have in common that they are fed with melt from one or more extruders. This melt frequently passes through a pre-distributor, by which the melt is divided over a larger number of melt pipelines. During their further travel through the blowing head the melt pipelines mouth in extrusion gaps, which by its flat, planar embodiment can convert the melt strand, showing a circular cylindrical cross-section, which exits the melt pipelines, into a film or a film layer. If a single-layer film is produced, a single such gap is required. For the production of a film with a number n of layers generally here n number of such gaps is needed as well.
After a mouthing of the melt pipeline in the extrusion gap, the melt pipelines convert into melt channels, which are formed by recesses in one or both of the boundary walls of the extrusion gap. The melt channels extend along the boundary walls of the extrusion gap, in order to distribute the melt in said extrusion gap. During their progression in the extrusion gap the melt pipelines increasingly taper until they finally convert entirely into the extrusion gap.
The publications EP 1055504 B1, DE20307412 U1, as well as U.S. Pat. No. 5,716,650 B show, among other things, the above-mentioned facts. When comparing the general design of the blowing heads shown in the three publications here general differences are striking, though:
U.S. Pat. No. 5,716,650 B shows a film blowing head, which essentially comprises a stack of round plates, which show a round recess in their center (round perforated disks). The exterior diameter and the diameter of the round recess are identical in the round plates of a blowing head. The stack of round plates, in the area of said round recess, is provided with an inner mandrel and shows an overall circular cylindrical shape. The melt is fed from the outside to the multi-layer blowing head via initially externally extending melt pipelines in the radial direction. The individual plates define the individual extrusion gaps, which form the respective film layer. The melt pipelines feed the melt to the gaps. After the gap is reached the melt pipelines convert into melt channels, which in the film gap point helically towards the center of the circular cylinder. Generally the melt pipelines are only formed by a groove in one of the two plates limiting the respective gap. U.S. Pat. No. 5,716,650 B shows however a blowing head, in which the channels are formed by grooves in both plates limiting the respective gap.
Over their path in the direction towards the center of the blowing head the helically extending grooves taper (their depth in the walls of the gap reduces) until the grooves end entirely. At the points the grooves end the melt has entirely converted into the gap. Over its remaining path through the gap the melt is further formed into its “new” planar shape. Finally the gaps forming the individual layers of the film end by mouthing in the multi-layer gap, which exists between the inner mandrel and the plates. By the individual layers of film mouthing here a melt flow develops, which already includes the layers of the future multi-layer film. The extrusion of this melt flow occurs by an annular gap, which is typical for film blowing heads. Film blowing heads of the above-described type, which are formed by a stack of plates, are frequently called “stack die,” as common in the English language.
EP 1055504 B1 shows such a stack die, which however comprises a few structural differences in its design in reference to the stack die of U.S. Pat. No. 5,716,650 B.
Individual gaps mouth in the multi-layer gap, which are guided in the radial direction of the blowing head from the inside and from the outside towards the multi-layer gap. Individual disks show conical forms.
In the context with such a film blowing head, frequently the use of a melt pre-distributor is recommended, which distributes the melt inside a closed component over several pipelines.
An alternative design of a film blowing head is shown in DE 203 07 412 U1. In these blowing heads the extrusion gap, forming the individual layers, already extends circular and cylindrical around the primary axis of symmetry of the blowing head, which is also circular and cylindrical.
Melt channels extend along this extrusion gap like helixes, which also taper in the direction towards the mouthing of the melt channel to a common melt channel at the upper axial end of the blowing head by their reduction in depth in the boundary walls of the melt channels until they entirely convert into said melt channel. Here, it must be stated that, contrary to many other documents of prior art, DE 203 07 412 U1 also shows melt channels formed by recesses in both boundary walls of the melt channels.
Obviously here, both by DE 203 07 412 U1 as well as by U.S. Pat. No. 5,716,650 B, the formation of striations or exudation marks in the film should be avoided. However, the measures suggested in these two publications cannot entirely prevent the formation of striations or exudation marks, so that professional users still need a solution for these two problems.